The victim was driving a UPS delivery van on the A39 between Higher Clovelly and Woolsery when he was flashed by a dark blue Ford Focus or Fiesta car.
When the victim pulled in to let the car pass a man got out of the Ford and approached the van.
The man demanded the van at knifepoint and the victim suffered minor cuts to his forearm. After the offender failed to get into the van he returned to his car and made off empty-handed.
Police are investigating the incident which happened between 11.40am and 11.50am on Wednesday 1 August.
They are seeking a suspect pictured in the e-fit; he is described as being of mixed race, around 6ft tall and with a muscular stocky build. He was wearing a dark green long sleeved top and dark blue jeans.
A police spokesman said: “This is a highly unusual incident and we are appealing to anyone who may have witnessed anything suspicious in the area at the time of the offence to contact us. We would like to hear from anyone who recognises the man in the image.”
Anyone with information about the incident is asked to contact police via email@example.com or by phone on 101, quoting crime reference CR/069629/18.